In certain applications, interference fasteners are in intimate contact with a structure in which they are placed. In some applications, it is desirable to have the presence of a minimal number of and/or small-sized spaces or voids, or the absence of spaces and voids altogether, between the fastener and the structure.
For example, numerous interference fasteners and accompanying sleeves are used on a single aircraft. In the case of a lightning strike on the aircraft, if a fastener/sleeve is not in intimate contact with a sidewall of a hole in the structure in which the fastener/sleeve is inserted, the instantaneous heat energy caused by the lightning ionizes the air in the voids between the fastener/sleeve and the sidewall, and creates arc plasma that blows out in the form of a spark. This is a very dangerous condition to have for an aircraft, especially if sparking occurs near a fuel tank.
With particular reference to aircraft structural components made from composite materials, such as carbon fiber reinforced plastics, these voids may be created during a process of drilling fastener holes in the composite material. Individual carbon fibers fracture at irregular angles and form microscopic voids between a fastener/sleeve and the hole. As the cutting tool wears down over time, there is an increase of surface chipping in the structure, which leads to an increase in the amount of uncut fibers or resin and delamination—also know as machining-induced micro texture.
When lightning strikes the surface of the composite structure, the current density tends to be higher around the metallic fasteners, which are commonly used to attach the various elements of the structure. These currents may create detrimental ignition sources by attaching to a fastener and flowing through the fastener to some point within the structure, which is disposed to arcing. In order to avoid this condition, the current must dissipate through the carbon fibers perpendicular to the fastener hole. As described above, if the fastener is not in intimate contact with side walls of the hole, the instantaneous heat energy ionizes the air in the voids and creates arc plasma that blows out in the form of a spark.
Additionally, electrically conductive coatings are used for a variety of applications, such as charge dissipation and radio frequency interference (EMI/RFI) shielding. The amount of direct current conductivity required is dependent upon the specific application. Electric charge buildup by dielectric substrates, such as fiberglass structures in frictional contact with other materials, can result in very large static voltages that may result in dangerous discharge sparks. The amount of surface resistivity required to effectively bleed off this charge and prevent sparking is usually rather low, 106 to 109 Ω/cm2.